So unless emacs uses this workaround for now, it will not have
functioning alpha transparency

[with non-compositing non-property-propagating wms and xcompmgr]

Well, not settable from within emacs. The "transset" (and more
full-featured derivative "transset-df") tools should still work though,
as IIRC they walk the window hierarchy up to one below the root and thus
set the property where xcompmgr expects it.

If a workaround were to be added back in to emacs, a similar walk would
at least be better than just assuming the immediate parent is the right
place to set it.

when used in combination with a large majority of window managers.

OTOH, it will still work without the workaround with the various
big-name-desktop-env default compositing window managers that likely
account for the majority of actual desktops. Of course I'd expect non-
big-name-desktops to be more popular with emacs users than users in
general though.

Perhaps there should be a push for this to be formally specified in
the EWMH. The proposal for it seems to be many years old, and the
practice widespread, so I don't see why this hasn't happened yet.

... And clients now have another, arguably much better (though somewhat
more work for the client) way of specifying much more fine-grained
(per-pixel) transparency, using an ARGB visual (AFAICS what "conky" you
mentioned does).

What I'd _like_ to do is to *:
(i) alter emacs display-engine/face-resolution to do alphablending, and
(ii) emacs itself to use an ARGB visual overall

((i) and (ii) are actually only loosely related, you could have (i)
without (ii) and vice-versa)

translucent region highlighting that doesn't obscure colored-background
faces but rather blends over them (only needs (i)),

etc.

* bearing in mind I do have not much time right now to actually do it,
and even if I (or someone else) did such a thing, it's not something
that is likely to be able to go in-tree for months right now (feature
freeze, and bidi being a large change in the general area), so it's not
of immediate use to you.